The invention relates to the irradiation of matter with a charged particle beam. In particular, it relates to a method and an apparatus for manipulating an electron beam of small cross section so that it can cover a relatively large area.
Various techniques have been developed to diffuse an electron beam (which usually has a diameter of about one millimeter) to irradiate areas having diameters exceeding 20 centimeters.
In Electromedica No. 3-4 (1977) pages 101 to 106, there is disclosed a linear electron accelerator (LINAC) in which a set of consecutive scattering foils creates an enlarged circular cross section with a homogeneous intensity distribution. The foils have a number of disadvantages: the average beam energy is decreased; the energy spectrum is widened; the energy level and/or the field size cannot be changed easily; and undesirable X-ray radiation is produced in both foils.
One way to prevent X-rays is to spread or scan the pencil-like beam along a fan axis perpendicular to the beam axis, i.e. by a magnetic or electrostatic lens, and to move the matter to be irradiated across the fan axis. This technique, which is described in more detail in U.S. Pat. No. 2,866,902, is used for sterilizing and preserving food, but has not yet been used for radiotherapy of humans. This is because it is not easy to ensure that a predetermined area of a laterally moved body is irradiated with an even intensity distribution.
The patient can be kept in a stationary position if the beam is transversely enlarged along both main axes. This is achieved, as disclosed in U.S. Pat. No. 3,120,609, by sending the beam through a quadrupole magnet. The magnet is designed so that the beam is defocused along one axis and along the other axis first focused and then, after the cross over of the beam particles, fanned out. Such an approach makes it difficult to obtain a homogeneous intensity distribution and, in particular, an accurate field limitation.
In Medical Physics 11 (1984) pages 105 to 127, section "Scanned Pencil Beams", there is mentioned a further alternative in which the electron beam passes two scanning magnets placed orthogonal to each other. By varying their magnetic fields, a raster or spiral scan can be provided. This scan technique is capable of providing treatment fields which are uniform and arbitrarily variable. Disadvantageous is however, that the beam needs a relatively long time to sweep the whole area and requires complex control and monitoring circuits to avoid "hot spots".
It is an object of this invention to provide a method and means for distributing the intensity of a charged particle beam over a relatively large area such that this area is irradiated with a substantially uniform current intensity.
It is another object of this invention to provide a method and means for distributing a charged particle beam so that the initial energy spectrum of the beam particles is not significantly altered.
It is a further object of the invention to provide a method and means for distributing a charged particle beam without creating detrimental X-rays.
It is yet another object of this invention to provide a method and means for distributing a charged particle beam so that the area to be irradiated and/or the energy level of the charged particle beam can be varied.
It is still another object of this invention to provide a method and means for distributing a charged particle beam in a mechanically and electrically simple manner.
It is a further object of this invention to provide a method and means for distributing a charged particle beam over a relatively large area so that the whole area is covered within a short time.
Still another object of this invention is to improve on the existing methods and means to enlarge charged particle beams.